In the prior art, phosphor fillers including phosphor particles are used in a broad field of applications, ranging from electro-luminescence to photo-luminescence devices. This rich applicability is due to the favourable physical properties of phosphor such as a high luminescence efficiency and lifetime as well as due to the presence of suitable emission colours in the optical emission spectrum.
A technological application of such phosphor fillers with increasing importance is the light emitting diode (LED), which comprises a LED-chip being electrically connected to a electrically conducting contact base. The LED-chip usually comprises a semiconducting p-n-junction, in which electrons and holes, which have been injected via a supply voltage, recombine under light emission. In order to direct the light emission into the operational direction of the LED-Chip, the LED-Chip is usually encapsulated by an optical dome made of transparent resin which, in turn, may include a phosphor filler by which the light emitting spectrum of the LED-Chip can be converted as necessary.
In particular, due to the development of blue emitting LED-chips and the use of such phosphor fillers, LED-devices providing a wide colour range can be obtained, including the so-called “white LED”, which can compete with conventional types of light sources in a broad field of applications such as traffic lights and signboards.
Generally speaking, such phosphor fillers may be based on different types of phosphor compounds, namely stable and unstable phosphor compounds. Stable phosphor compounds may e.g. comprise members of the garnet family, preferably (YGd)3Al5O12 including Ce3+-impurities. Unstable phosphor compounds may e.g. comprise SrGa2S4:Eu2+, SrS:Eu2+, (Sr,Ca)S:Eu2+, ZnS:Ag.
The advantage of a phosphor filler in the form of stable phosphor compound particles is that it is not sensitive to moisture which would, in turn, reduce the reliability of the electrical device, like an LED-Chip encapsulated in an epoxy dome comprising such a phosphor filler.
However, it is also known in the prior art that the performance of devices with unstable phosphor compound can be improved, too, by coating the phosphor compound material, i.e. the outer surface of the individual unstable phosphor compound particles, with a protective coating film. In particular, said unstable phosphor compound particles may be coated with an inorganic coating film including a moisture-proof barrier material such as aluminium oxide (Al2O3), zinc sulphide (ZnS), silicon nitride (Si4N3) or the like. In case of such fillers based on unstable phosphor compounds, the inorganic coating film on the individual-phosphor compound particles provides for a chemical and photochemical degradation protection of the phosphor compound.
In the light of the foregoing explanations, the term “phosphor filler” refers, in the following description, to a plurality of phosphor filler particles which are either stable phosphor compound particles or unstable phosphor compounds particles coated with an inorganic moisture-proof coating film.
From U.S. Pat. No. 4,585,673, a method for forming a protective coating film on unstable phosphor compound particles is known, wherein the protective coating film is formed by a gas-phase chemical vapour deposition (MOCVD=“metal organic chemical vapour deposition”) on the phosphor compound particles which are suspended in a fluidised bed which is maintained in a temperature gradient, said protective coating being a refractory oxide such as aluminium oxide.
U.S. Pat. No. 6,001,477 discloses a method for providing on the surface of individual unstable phosphor compound particles a continuous, non-particulate coating of a metal or metalloid compound such as silicon or boron by means of a reaction between the metal or metalloid and a polymer capable of chelating ions of the metal or metalloid. The resulting coating (e.g. a BA-PVM/MA coating) is chemically adhered to the phosphor compound particles which exhibits improved lumen maintenance when applied to the inner surface of a lamp envelope.
U.S. Pat. No. 5,985,175 discloses a method for providing on individual unstable phosphor compound particles a continuous, non-particulate coating of boron oxide to enhance the quantum efficiency of the phosphor compound particles under ultraviolet (UV) and vacuum ultraviolet (VUV) excitation. The method involves reacting a boron-containing precursor with an oxidizing gas in a fluidised bed of phosphor particles.
Furthermore and more generally, EP 0 539 211 B1 discloses a method for production of a microcapsule type conductive filler, wherein this conductive filler is dispersed in an epoxy type one-component adhesive agent.
A possible structure of a phosphor filler 100 according to the prior art is schematically illustrated in FIG. 2a. The phosphor filler 100 comprises a plurality of unstable phosphor compound particles 101, each of the phosphor compound particles 101 being coated with an inorganic coating film 102. The inorganic coating film 102 consists of a suitable moisture-proof barrier material such as e.g. aluminium oxide (Al2O3) and has a thickness in the range of about 3 to 4 μm.
If the thickness of the coating film 102 is large, the coating film 102 provides a significant deterioration of the optical transmissibility. On the other hand, if the thickness of the coating film 102 is low, the spacing between neighbouring phosphor compound particles 101 is relatively small. Consequently, the probability of light symbolized by light beams 103, which is e.g. emitted by a LED as described below with reference to FIG. 2b, to be re-absorbed by surrounding phosphor compound particles 101 is high and, therefore, the brightness obtained in a LED using this kind of phosphor filler is low.
A typical LED 200, as schematically illustrated in FIG. 2b, comprises a LED-chip 201, which is mounted on a first electrically conducting frame 202. Said first electrically conducting frame 202 is provided with a reflector cup 202a including a recess in which the LED-chip 201 is mounted. At least two electrodes (not shown), which may be surface mounted electrodes, are attached on said LED-chip 201, one being electrically connected by means of a first wiring 203 to the first electrically conducting frame 202, and the other being electrically connected by means of a second wiring 204 to a second electrically conducting frame 205.
The LED-chip 201 is covered by a drop 206 containing a mixture consisting of epoxy and a phosphor filler dispersed therein, said drop 206 filling almost the whole recess of the reflector cup 202a. The phosphor compound particles of the phosphor filler may be coated with a coating film including a moisture-proof barrier material such as aluminium oxide (Al2O3), i.e. they may form a structure as described above with respect to FIG. 2a. 
Furthermore, the major upper part of the first and second electrically conducting frames 202 and 205 as well as the whole arrangement formed by the LED-chip 201 covered by the drop 206 and the wirings 203 and 204 are encapsulated by an optical dome (or optical lens) 207 formed of transparent epoxy.
The LED 200 can e.g. be operated as a white light emitting diode, wherein phosphor compound particles in the drop 206 re-emit a broad band of yellow, yellow-green or red-green light with unabsorbed blue light from the LED-chip 201.
Two common methods for forming a LED device are schematically illustrated in FIG. 3. These methods are generally referred to as the “pre-mix method” (FIG. 3a) and the “pre-dep method” (FIG. 3b).
In the so-called “pre-dep method”, as can be seen in FIG. 3b, a LED-chip 301 of a LED device 300 is placed inside a reflector cup 302 of a metal base 303 in a first step. Then the LED-chip 301 is electrically connected, by means of wirings 304, to the metal base 303. In the next step, a drop 305 containing a mixture of phosphor compound particles 306 and epoxy 307 is filled into the reflector cup 302 to cover the LED-chip 301. Finally, the whole structure of the drop 305 covering the LED-chip 301, the wirings 304 and the metal base 303 is over-moulded with epoxy to form a transparent optical dome 308.
In contrast to this method, the so-called “pre-mix method” prevents a procedure of covering of the LED-chip 301 in two steps. To achieve this simplification of the manufacturing process, the LED-chip 301 is over-moulded, as can be seen in FIG. 3a, in only one step by an optical dome 309 containing a pre-mixed mixture of phosphor compound particles 310 and epoxy 311.
Accordingly, whereas the pre-mix method of FIG. 3a simplifies the manufacturing process, the pre-dip method of FIG. 3b provides, due to the completely transparent optical dome 308, a more efficient light extraction from the LED-chip 301.
However, optical devices such as a light emitting diode (LED) including phosphor fillers according to the prior art, i.e. stable or unstable phosphor compound particles being coated with none or only one protective coating film, respectively, wherein the coating film consists of e.g. aluminium oxide, exhibit several shortcomings for the following reasons:                (1) A significant basic problem of prior art LED devices of the type described above is that the phosphor filler, i.e. the individual phosphor compound particles tend to agglomerate. This problem is observed and equally valid for all type of phosphor fillers discussed above, that is for stable phosphor compound particles as well as for unstable phosphor compound particles which are coated with an inorganic moisture-proof coating film. Such an agglomeration leads, however, to a number of drawbacks in the operating characteristics of the LED device, such as uneven spectral and brightness distribution of the emitted light over the emitting surface of the device, loss of brightness of the LED device based on re-absorption effects between neighbouring agglomerated phosphor particles, etc.        (2) The prior art LED devices containing unstable phosphor compound particles coated with an inorganic film as phosphor filler exhibits a relatively poor light extraction efficiency. In other words the amount of light emitted by such a device compared to the amount of light which would be emitted by a device which does not comprise such a phosphor filler is significantly reduced. This is based on the fact that the refractive index of the inorganic coating film, such as aluminium oxide, differs from the refractive index of the epoxy resin, resulting in that the light entering and passing through the encapsulating dome experiences several times total reflection at the inorganic coating film—epoxy—interfaces and, thereby, captured within the dome.        (3) In the prior art LED device 200, the unstable phosphor compound particles in drop 206 exhibit a relatively large sensitivity to moisture, which may enter in the drop 206 and attack the unstable phosphor compound particles situated therein. Consequently, the LED 200 is subjected to aging effects, so that the reliability of such a prior art LED is relatively low. This effect is especially disadvantageous in applications such as traffic lights or signboards, which generally require lifetimes of the used optical device of more than 105 h.        (4) If unstable phosphor compound particles are used in the drop 206 of the LED 200 which are individually coated with a protective coating film consisting of e.g. aluminium oxide, the protective coating reduces the optical transmission of the drop 206 and thereby the brightness of the LED 200. Accordingly, the thickness of this protective coating and, consequently, the protection of the unstable phosphor compound particles are limited.        
Accordingly, the performance of LED's using prior art phosphor filler is insufficient particularly with respect to the agglomeration problem, but, at least in case of unstable phosphor compound particles also with respect to light extraction, i.e. the brightness of the LED. Accordingly, the reliability of the known LED devices is low.